It is a percentage increase. It is the same for all 3 stats. Your stat pool can be calculated as follows
The Base value at V16 is 8744 for Health and 7958 for Magicka and Stamina. Attribute points is the number of points spent in Health, Magicka or Stamina multiplied by 122 for Health and 111 for Magicka and Stamina. CPI is a cumulative percentage increase due to points spent in the corresponding constellation and can be calculated as follow
The Lord Mundus gives 1452 Health. The Mage and Tower Mundus gives 1320 Magicka and Stamina, respectively. Divines is the sum of divines. For example 4 pieces of green equipment with divines (4.5% each) means Divines = 1.18. Limited testing on the PTS suggest that Divines is rounded to 2 decimal places.
Example
I have a V16 Breton Sorcerer on the PTS. My gear gives me 7924 Magicka. This includes enchantments and gear bonuses. I have 64 points in Magicka, thus Attribute Points is 7104. I have 100 Champion Points in the Mage constellation which means that CPI is 1.134. I am using the Crown Fortifying Meal which gives 3617 Magicka. Skills is 1.31 (8% Bound Aegis, 5% Inner light, 2% Magicka Controller, 10% Gift of Magnus, 6% Undaunted Mettle). Putting this all into the formula
My in-game Magicka pool in the PTS is 38899.